In mobile communications, when a signal is transmitted by an antenna it is subject to the effects of multipath and, as viewed from the receiver, the transmission comprises a plurality of radio waves simultaneously propagated to the same receiver. Multipath, i.e., interference occurring between the plurality of radio waves, will cause the radio waves to have different propagation delay times to the receiver. This is because of varying propagation path lengths of the radio waves. In DS-CDMA (Direction Sequence-Code Division Multiple Access) communication, information data is spread with spreading codes at a higher rate with a shorter period than the propagation time. For this reason, radio waves with different propagation delay times can be separated and extracted.
Movements of a mobile station change the positional relationship between the mobile station and a base station, and hence the delay profile, which is signal distribution with respect to delay time, also changes over time. Signals propagating along paths other than paths in which a base station is directly viewed from a mobile station are varied in accordance with the Rayleigh distribution.
In DS-CDMA communication, a plurality of path signals separated in terms of time with different propagation delay times and changing in accordance with the Rayleigh distributions are in-phase combined (rake combined) to obtain a diversity effect, thereby improving reception characteristics. However the relative positional change of a mobile station with respect to a base station causes variations in delay profile as well as variations in delay time of paths to be rake combined. Therefore, in a mobile communication environment, a receiver requires a multipath search, tracking function for following variations in delay profile to allow instantaneous rake combination of the plurality of paths that can provide maximum signal power.
In a conventional method of allocating multipath to fingers, a delay profile for propagation paths is first measured with a matched filter or sliding correlator. Upper correlation peaks are detected from the measured delay profile, the number of the detected correlation peaks being equal to the number of fingers possessed by a Rake receiver. A detected path timing is allocated as despread timing for each finger. A problem can occur in the case of a receiver having N fingers, where N is an integer, is that the delay profile has a Nth correlation peak and a (N+1)th correlation peak with substantially equal powers, the Nth and (N+1)th path timings frequently interchanging depending on variations in propagation path. This causes frequent switching of allocated path timing (despread timing) in one of the fingers, resulting in significantly deteriorated reception characteristics.
US 2003/0026233 A1 discloses a method of, and CDMA receiver for, providing hysteresis in detection of path timing by multiplying the delay profile by a weighting coefficient. The CDMA receiver includes calculating means, operational means and search means. The calculating means calculates a state weighting coefficient based on the present path allocation to a plurality of finger receivers. The operational means performs a predetermined operation between the calculated state weighting coefficient and a delay profile. The search means searches for paths based on the weighted delay profile. In implementing the method disclosed, state weighting, calculated on the basis of the present state of allocation to rake fingers, is performed for a measured delay profile in a multipath search unit of a Rake receiver, which provides hysteresis for path switching level of fingers to make it possible to prevent frequent switching of paths which leads to deteriorated characteristics. More particularly path switching will only take place if the correlation power of a new candidate path exceeds an existing path by a factor α, where typically a is equal to approximately 1.5. In this way it is possible to provide hysteresis for replacement of paths and to prevent frequent replacement of paths. The cited method can also protect path timing for finger receivers even when a path is temporarily lost due to shadowing or the like, resulting in improved reception characteristics.
US 2004/0156423 A1 discloses a method of channel gain estimation in a Rake receiver using complex weight generation (CWG) algorithms. This citation is concerned with correcting a frequency offset between a base station (BS) and wireless transmit/receive units (WTRU), which frequency offset translates into a phase shift over time and must be estimated and corrected in the WTRU or else a significant loss in performance will occur. In the case of the WTRU being carried in a vehicle the speed of the vehicle will introduce time and frequency shift which could cause severe performance degradation within the Rake receiver. However, it is possible to estimate the phase shift and compensate for it in the CWG process since the phase shift is due to a constant frequency offset.
In 3GPP (Third Generation Partnership Project) standard specification there is no method proposed to define finger assignment algorithm. The number of fingers to be assigned is in general limited (for size and power consumption reasons). When a path is disappearing because its power falls below a specified value, the associated finger is removed so as to be available to another stronger path. However in the opposite situation when a powerful path appears and all the fingers having been assigned already, finger replacement makes sense and can be critical. Although hysteresis is desirable to avoid frequent switching of paths, it can be problematic in slowing down a finger replacement process.